Suspected militants in southern Pakistan have destroyed at least 27 tankers carrying fuel for foreign troops in Afghanistan.The incident came the day after the Pakistan government blocked supply routes to its neighbour in an apparent protest at a Nato air raid which killed three Pakistani troops.The convoy of tankers attacked on Friday is believed to have been heading to a second crossing in the southwest of the country which was still open. It is not yet clear if the vehicles had been diverted because of the border closure.